 Shor-code phase-flip syndrome

Group the nine physical qubits into three blocks and write $B_r$ for the product of three Pauli X gates in block $r$. The commuting stabilizer generators $B_1{B}_2$ and $B_2{B}_3$ have eigenvalue $+1$ on both logical codewords. A Pauli Z gate in blocks one, two or three gives syndromes $(-,+)$, $(-,-)$ or $(+,-)$ respectively. Apply a Z gate to any one qubit of the indicated block to recover the logical state. Z errors at distinct locations within a block act identically on the code, so this correction is degenerate.